The usual way to form and interconnect cells in thin film solar panels involves sequential layer coating and laser scribing processes. To complete the structure three separate coating processes and three separate laser processes are usually required. It is usual to perform these processes in a six step sequence consisting of a laser step following each coating step as described below:                a) Deposit a thin layer of the lower electrode material over the whole substrate surface. The substrate is usually glass but can also be a polymer sheet. This lower layer is often a transparent conducting oxide such as tin oxide (SnO2), zinc oxide (ZnO) or indium tin oxide (ITO). Sometimes it is an opaque metal such as molybdenum (Mo).        b) Laser scribe parallel lines across the panel surface at typically 5-10 mm intervals right through the lower electrode layer to separate the continuous film into electrically isolated cell regions.        c) Deposit the active electricity generating layer over the whole substrate area. This layer might consist of a single amorphous silicon layer or a double layer of amorphous silicon and micro-crystalline silicon. Layers of other semiconducting materials such as cadmium telluride and cadmium sulphide (CdTe/CdS) and copper indium gallium di-selenide (CIGS) are also used.        d) Laser scribe lines through this active layer or layers parallel to and as close as possible to the initial scribes in the first electrode layer without damaging the lower electrode material        e) Deposit a third, top electrode layer, often a metal such as aluminium or a transparent conductor such as ZnO, over the whole panel area.        f) Laser scribe lines in this third layer as close to and parallel to the other lines to break the electrical continuity of the top electrode layer.        
Generally, it is necessary to move the substrate between different environments (eg a vacuum or atmospheric environment) between each of these steps.
This procedure of deposition followed by laser isolation breaks up the panel into a multiplicity of separate long, narrow cells and causes an electrical series connection to be made between all the cells in the panel. In this way, the voltage generated by the whole panel is given by the product of the potential formed within each cell and the number of cells. Panels are divided up into typically 50-100 cells so that overall panel output voltage is typically in the 50 to 100 Volt range. Each cell is typically 5-15 mm wide and around 1000 mm long. A thorough description of the processes used in this multi-step solar panel manufacturing method is given in JP10209475.
Schemes have been devised to simplify this multi-step process of making solar panels by combining some of the separate layer coating steps. This reduces the number of times the substrate has to be moved from a vacuum to an atmospheric environment and hence is likely to lead to improved layer quality and increased solar panel efficiency. U.S. Pat. No. 6,919,530, U.S. Pat. No. 6,310,281 and US2003/0213974A1 all describe methods for making solar panels where two of the 3 required layers are coated before laser scribing is performed. The lower electrode layer and the active layer (or layers) are deposited sequentially and then both layers are laser scribed together to form a groove that is then filled with an insulating material. For U.S. Pat. No. 6,310,281 and US2003/0213974A1 it is proposed that this groove filling be performed by ink jet printing. Following the groove filling, the interconnection procedure is as described above with a laser scribe through the active layer, deposition of the top electrode layer and a final scribe of the top electrode layer to isolate the cells.
A scheme has also been proposed where all three layers are coated before any laser scribing is performed. WO 2007/044555 A2 describes a method for making a solar panel where the complete three layer stack is coated in one process sequence following which laser scribes are made into and through the stack. The laser scribe process is complex as it consists of a single scribe with two different depths. On a first side of the scribe the laser penetrates the complete three layer stack right through to the substrate in order to electrically separate the lower electrode layer to define the cells while on the second side of the scribe the laser only penetrates through the top and active layers to leave a region where a ledge of lower electrode layer material is exposed. Insulating material is applied locally to the first side of the scribe that penetrates to the substrate so that the insulating material covers the edge of the lower electrode layer and the edge of the active layer on the first side of the scribe. Following this, conducting material is deposited into the scribe so that it bridges the insulating material previously applied and connects the top electrode layer on the first side to the ledge of lower electrode material on the second side.
The process described in WO2007/044555A2 is complex and requires careful control. Debris generated during the second stage of the dual level laser scribe process is likely to deposit on the adjacent top surface of the ledge of lower electrode material leading to poor electrical connection. A high level of control is needed to ensure that the insulating material is placed in exactly the right position on the first side of the scribe and no material is deposited on top of the ledge of lower electrode material. Extreme accuracy is needed to ensure that the conducting material is placed correctly and does not contact the top electrode on the second side of the scribe. For all these reasons it is unlikely that cell connections can be made with high reliability by this method.
Hence, there remains a requirement for a new cell formation and interconnection process for solar panels and the like that starts with the full stack of three layers but proceeds to make the cell interconnections in a way that is fast, simple and reliable.
Such a process will also be applicable to the formation and series interconnection of cells for the manufacture of other thin film devices such as lighting panels or batteries. Like solar panels, such devices consist of a lower electrode layer, an active layer and a top electrode layer all deposited on a rigid or flexible substrate. Operation at voltages higher than the fundamental single cell voltage can be achieved by dividing the device into multiple cells and connecting the cells in series. The laser and ink jet cell formation and interconnection apparatus proposed here is suitable for such an operation.
For lighting panels, the upper and lower electrodes are likely to be of similar materials to those used for solar panels (eg TCOs or metals) but the active materials are very different. In this case, active layers are most likely to be organic materials but inorganic materials are also possible. Active organic layers are either based on low molecular weight materials (so called OLEDs) or high molecular weight polymers (so called P-OLEDs). Hole and electron transport layers are usually associated with the active light emitting layers. For these lighting panels, operation is at low voltage and all layers are thin and hence the interconnection process described herein is ideal for dividing the panel into cells and connecting these in series to allow operation at a substantially higher voltage
For thin film batteries the layers are often more complex. For the case of a thin film battery based on Lithium ion technology, the lower layer has two components—a metal layer for current collection and a Lithium Cobalt Oxide (LiCoO3) layer that functions as a cathode. The upper layer also has two components—a metal layer for current collection and a Tin Nitride (Sn3N4) layer that functions as an anode. In between these two layers is the active layer—a Lithium Phosphorous OxyNitride (LiPON) electrolyte. For such batteries, operation is at low voltage and all layers are thin and hence the interconnection process described herein is ideal for dividing the battery into cells and connecting these in series to allow operation at a substantially higher voltage
The applicant has in an earlier (unpublished) patent application proposed a method and apparatus for dividing a thin film solar panel into separate cells and connecting these cells electrically in series. The apparatus used a process head that combines three laser beams and two fluid nozzles. All five of these components on the process head addresses a single interconnect structure between two adjacent cells so that a single passage of the head across the solar panel creates a single interconnect structure. Other interconnect structures are then formed by stepping the process head by distances equal to the cell width, or multiples of the cell width, in the direction perpendicular to the interconnect direction and traversing the process head across the panel again. The present invention provides an improvement of the method and apparatus proposed in the earlier application.